Cheap Chain Keeper?
 

I'm mostly a city-commuter, and I recently switched to a single-speed crank. I love it, but it seems like when I shift into my top gear (7th), it tends to throw the chain off the crank quite often, maybe about 15-20% of the time unless I start pedaling very slowly as I shift there. I should note, for what it's worth, that the chain seems to almost always be thrown off to the pedal or away from the frame, rather than being thrown inward.

I found the "Paul Components Chain Keeper"[1], and it seems to be exactly what I need, but the $55 MSRP (or even $41 at JensonUSA) seems a little steep. Is there anything cheaper out there that would do the trick? I've been thinking about trying to somehow fix a front derailleur in-place so that it keeps the chain there, but I don't know whether it would be too wide (perhaps I could make it more narrow with pliers or something?).

Ideally I suppose I would use a part intended for what I'm doing, but I'm just kind of a cheap skate and trying to brainstorm for ideas. Thanks for any input.

[1] http://www.paulcomp.com/chainkeeper.html

Personally, I'd take apart an old cheap front derailleur so I had just the cage and tube clamp, weld them together at the requisite reach using whatever tubing or barstock I had lying around, polish it all up and call it done. Could even make it adjustable by welding nuts to the cage and clamp and running bolts through them. But you may not be looking quite so involved a solution.

There's no good reason your chain should unship that often. Single chainwheel 5+ gear bikes have existed for years without chain keepers. You might want to check your chain length as well as how worn it is. A worn chain does not sit a tightly on the chainwheel and the extra lateral give can sometimes cause it to "whip" off more easily. Other than that I would say a front derailleur would work fine.

One thing that will probably help is replacing the chainring with one intended for single speed. These don't have the shifting gates which are often the start of the derailment. This might be something to file away for when your current chainring wears out, rather than spending $$$ now.

If you still have your old front derailleur, you can probably mount it and use the limits to set the trim where it'll do the job. Depending on the chainring position your inner limit may be too short, but that's easily fixed with a longer screw.

You might want to pinch the front a bit tighter and spread the back of the cage so it'll do the job without needing any trim as the chain feed angle changes with gear changes. Note many modern derailleur cages don't handle bending very well and might crack. Older front derailleurs that might be scavenged of junk bikes tend to have more malleable cages suited to this purpose.

Did you install a bottom bracket with a shorter spindle when you installed your single-speed crank? You should have.

If you decide to go with a 'SS specific' ring, Surly makes stainless steel ones that are good. I use one for my middle ring on my tourer (for longevity).

Negative. It seems though that when the chain is throw off, it comes off on the right side of the crank (away from the frame). It seems that a shorter spindle would bring the crank closer to the frame and so might not necessarily help for this particular problem. I will look into it, though. Is it best practice to do this so that the chain will not be so stressed? I'm somewhat new to bicycle mechanics, so please forgive me if I misunderstand this point.

It seems like this component serves to keep the chain from being thrown off the granny gear or toward the frame. I should have specified in the original post that my chain tends to be thrown off toward the pedal and away from the frame.

Yup. It keeps the chainline as straight as possible so the chain is less tempted to jump off the ring when it's shifted to the extremes in the back. Basically, the ring should be as perfectly in line with the center of your cogs as possible.

the best chainkeeper is a front dérailleur. too bad you took it off.

Just an 'outside the box' thought--what about a bashguard sized large enough to keep the chain from coming off toward the pedal?

http://www.jensonusa.com/store/produ...eed+Guard.aspx

you can even buy 2 and some long chainring bolts and set them up like this:

http://www.peterwhitecycles.com/Cyclo-Cross.htm

I like both of those ideas, and I think both of those options would look much better asthetically, in my opinion, than having something mounted on the seat tube. The Shimano chainring guard [1] seems to be the cheapest option, at $10. The challenge now, I think, is to find something similar that fits my 5-bolt, 144mm BCD track crank. If anyone knows of one, I'd appreciate any links.

Thanks again everybody, for all the help and suggestions.

[1] http://www.jensonusa.com/store/produ...ing+Guard.aspx

Oh, you did mention that already. Sorry about that...

If it is a standard (double) 144bcd crank, could you mount the ring you use on the 'inner', and use a larger 'outer' ring as a 'keeper' (not to shift to)?

Just noticed you said 'track crank'--not too familiar with them--are they set up where you can only mount ONE ring?

Actually I hadn't. After I saw your post I edited the original post to clarify for any new readers. I should have left an edit-reason on the original post specifying that. My bad on that. :)

It looks like I could do this. The double-ring crank I replaced seems to have one chainring mounted on the inner-side of the spider and one on the outer-side. It looks like perhaps if I used longer 'bolts' (I'm not sure what these are called on the crank; they look similar to rivets) I should be able to do this. I'm not sure; here are some photographs if they are any help:

http://bikeisland.com/images/xrpcrank80.htm

By looking at the 'inside' and 'outside' views of this crank, it looks like you could mount the ring you want to use on the 'inner' pos., and some larger ring on the 'outer' as a 'chain stop' to that side. Good luck.

BTW I think they are called chainring bolts.

You'll want to do whatever is needed to locate the chainring as close as practical to be in line with the middle sprocket of your cassette or freewheel. If that means mounting the chainring to the inside of the crank arm spider then so be it. You can check this by laying a long straightedge on the face of your chainring so it extends back to the rear sprockets. You want it to line up with the middle of the stack or pretty close to it.

Even though the chain is being thrown off to the outside it could still be due to some oddball way that the chain is hooking the shift ramps on your ring. Switching to a ring without shift ramps or cut away shift teeth would go a long way to reducing the lifting and dumping of the chain.

An update:

After reading the 'chain length' section of Sheldon Brown's derailleur adjustment page, I unlinked my chain and measured the way he recommended, and discovered I had an additional 3/6 links in my chain. I've removed them and will be taking my bike out for a short ride this evening and a longer ride tomorrow. Hopefully a tighter chain will make a difference. I haven't got a straight-edge handy, but looking down as straight as I can, the chainring seems lined up pretty well with the center of the cassette in the back.

I'm not sure what shift ramps are or how to take a closer look at them. Does anyone know of a good diagram of the various parts of a chainring?

Thanks again, everyone, for all the suggestions. I'll let you guys know how things work out.